1. Field of the Invention
The invention relates to a machining head for a laser machining apparatus with which workpieces can be welded, cut, or machined in some other way.
2. Description of Related Art
Laser machining apparatuses ordinarily include a laser radiation source which may be, for example, a Nd:YAG laser, a fibre laser, a disk laser or a CO2 laser. A laser machining apparatus further includes a machining head, which focuses the laser radiation generated by the laser radiation source in a focal spot, and a beam-feed device which feeds the laser radiation generated by the laser radiation source to the machining head. The beam-feeding device may in this case include optical fibres or other light guides and/or one or more deflecting mirrors with plane or curved surfaces. The machining head may have been fastened to a mobile robot arm, whereas the laser radiation source is located outside the robot.
For the purpose of focusing the laser radiation in a focal spot, as a rule the machining head contains focusing optics. The latter include, besides lenses and/or mirrors, also one or more interchangeable protective discs which protect the sensitive optical elements of the focusing optics against contamination. The contamination may, in particular, be caused by splashes of material arising at the machining point or by smoke.
Above all when the laser radiation has a relatively low beam quality, as a rule it is fed to the machining head as a collimated beam with a relatively large diameter (20 mm to 100 mm). Laser radiation with higher beam quality, such as is generated, in particular, by fibre lasers and disk lasers, can also be fed to the machining head via an optical fibre. At a fibre plug the laser radiation emerges with relatively little divergence and is then collimated by a collimating lens in such a way that a beam with a 1/e2 diameter from about 15 mm to 20 mm arises.
Above all in the last-mentioned case, i.e. in the case of lasers with high power and good beam quality, locally very high intensities appear in the focusing optics. Particularly when the focusing optics contain lenses and other refractive optical elements such as protective discs, the unavoidable residual absorption in the lens materials that are used has the result that the elements heat up. This is accompanied by a change of shape as a consequence of the thermal expansion. In this way, even protective discs, which at room temperature act optically as a plane-parallel plate, may have a collecting action after the heating.
By virtue of the heating, the refractive power of the optical elements in question consequently changes, which has an effect on the shape and, above all, on the axial position of the focal spot generated by the focusing optics. Measurements have shown that the focal spot, particularly in the start-up phase, i.e. after the start of the laser machining, is displaced by several millimeters (typically 5 mm to 15 mm) in the axial direction. The temporal progression and the final value of the displacement depend on the beam power, on the beam quality and on the thermal properties of the optical components. Only when after several seconds or even several minutes a steady state has been attained, in which the distribution of heat in the optical elements of the focusing optics no longer changes appreciably, does the position of the focal spot remain constant to some extent.
By virtue of the unintended displacement of the focal spot, the workpieces may no longer be machined in the desired manner. If the workpiece is located outside the focal spot, the requisite energy densities for fusing metals, for example, are no longer attained, leading to the interruption of cutting procedures and, in the case of welding, to seam defects.
It is in fact known in the state of the art to monitor the machining region on the workpiece with the aid of individual sensors or cameras. However, as a consequence of the strong emissions of light in the region of interaction with the laser radiation it is difficult to register, unambiguously and in real time, changes of position of the focal spot with the requisite accuracy and independently of the machining process, and thereby to avoid machining defects.
Even when the focusing optics contain mirrors for focusing instead of lenses, changes of position of the focal spot may occur, particularly in the start-up phase. A (to begin with, small) portion of the laser radiation is not reflected by the reflecting coating of the mirror but penetrates into said coating and the underlying mirror substrate, which, for example, may consist of glass or copper, and is absorbed there. As in the case of lenses, the heating of the reflecting coating leads to a detuning of the layered system, which results in an increased absorption. Also in the case of mirrors the focal length is shortened, but much more quickly and much less than in the case of transmissive optical components, since the thermal conductivity of the mirror materials that are ordinarily used (e.g. copper) is very high. Besides the axial displacement of the focal spot, the high-energy laser radiation can also induce other imaging errors in the optical elements of the focusing optics. Such imaging errors may lead to a blurring of the focal spot, which likewise has a disadvantageous effect on the quality of the machining.
From JP S61-137693 A a refractive-power-measuring device for a laser machining apparatus is known, wherein a source of measuring light directs a collimated beam of measuring light via a plane deflecting mirror onto a converging lens of focusing optics. After passing through the converging lens the measuring light is focused in a focal plane of the converging lens, in which a point diaphragm has been arranged. Downstream of the pinhole diaphragm a light sensor has been arranged which registers the intensity of the measuring light that has passed through the pinhole diaphragm. If as a consequence of a heating of the converging lens the focal length thereof changes, the quantity of light registered by the sensor decreases. The axial positions of the pinhole diaphragm and of the converging lens are repositioned in a manner depending on the sensor signals. Since the respective position of the focal point at which the measuring light is focused always has to be sought anew in the event of a change in the focal length of the converging lens, the control system reacts relatively sluggishly.
From JP H02-204701 A a system is known with which the shape of a mirror that has been exposed to a laser beam can be kept constant independently of the thermal loading thereof. For this purpose, measuring light that was reflected from a specular surface is registered by a photodetector. Depending on the measuring signals, piezoelectric elements that have been fastened on the reverse side of the mirror are driven in such a way that they cancel a thermal change of shape registered by the measuring light.
From EP 2 216 129 A1 a laser machining head is known with integrated sensor device for monitoring the focal position. In the machining head a small portion of the laser radiation is coupled out of the collimated beam path between two converging lenses with the aid of a beam splitter and is coupled obliquely into the beam path via a collecting mirror in such a way that it passes through one of the two converging lenses of the focusing optics and an adjacently arranged protective disc. The collecting action of said mirror and of the converging lens has been established in such a way that the portion of the laser radiation coupled out for the measurement is focused, after passing through the converging lens and the protective disc, in a light sensor which takes the form of a CCD area sensor. If the refractive power of the converging lens and of the protective disc changes, the size of the focal spot on the sensor changes. Depending on the intensity distribution on the sensor, a traversing motion of the other converging lens is driven, in order to correct the position of the machining point (tool center point, TCP).
A disadvantageous aspect of this known measuring arrangement is that intense retroreflections from the workpiece can likewise, by virtue of repeated retroreflection on the input-side converging lens, get onto the light sensor and thereby falsify the results of measurement. In addition, the measuring device requires a relatively large additional construction space, although it is not capable of registering all the optical elements of the focusing optics.
From DE 10 2011 054 941 B3 a machining head of a laser machining device is known wherein a portion of the laser radiation that is reflected from the last or penultimate optical element of the focusing optics is coupled out of the beam path of the laser radiation via an outcoupling mirror and is focused in an image sensor via a converging lens. If the refractive power of the focusing optics changes as a consequence of heating, the focal point of the reflected laser radiation is also displaced. Since, however, light is also reflected from the workpiece to be machined, it is difficult to detect the weak retroreflection of said optical element among the large number of other reflections.